1. He writes.
2. He is writing.
3. He has written.
4. He has been writing.

The sentence 1 is said to be in the present indefinite or simple present tense because it just makes a simple statement about an action – the action of writing – without indicating definitely whether the action is complete or note.

The sentence 2 shows that the action of writing is continuing for some time in the present and hence the tense of the verb is called present continuous.

The sentence 3 shows that the action of writing has been completed or perfected at the time of speaking and hence the tense of the verb is called present perfect.

The verb in sentence 4 is said to be in the present perfect continuous tense because it indicates that the action which started sometime in the past has gone on continuously and is still going on. It is not yet complete.

Past Tense

Just as the present tense has four forms, the past tense also has four forms: